1. Field of the Invention
This invention relates to a direct conversion receiver which can be used in a mobile communication apparatus such as a PHS (Personal Handy Phone) telephone set, a portable telephone set, or a pager.
2. Description of the Related Art
Japanese published unexamined patent application 6-54005 discloses a direct conversion receiver having an FSK (frequency shift keying) demodulator. In the direct conversion receiver of Japanese application 6-54005, a received radio-frequency (RF) FSK signal carrying a digital information signal is fed to first and second mixers. The first mixer down-converts the RF FSK signal in response to a first local oscillator signal. The second mixer down-converts the RF FSK signal in response to a second local oscillator signal having a quadrature relation with the first local oscillator signal. The first local oscillator signal is outputted from a voltage controlled oscillator which serves as a local oscillator. A 90xc2x0 phase shifter converts the first local oscillator signal into the second local oscillator signal. The output signal of the first mixer is processed by a first low pass filter, being converted thereby into a baseband I (in-phase) signal. The output signal of the second mixer is processed by a second low pass filter, being converted thereby into a baseband Q (quadrature) signal. A demodulation circuit recovers the digital information signal from the baseband I signal and the baseband Q signal. The demodulation circuit outputs the recovered digital information signal.
In the direct conversion receiver of Japanese application 6-54005, one of the baseband I signal or the baseband Q signal is converted into a signal voltage by a frequency-to-voltage converter. The signal voltage is compared with a reference voltage by a voltage comparator. An in-phase/opposite phase discrimination circuit compares the phase of the output signal of the voltage comparator and the phase of the output signal of the demodulation circuit to decide whether the phases are equal or opposite to each other. When the phases are equal to each other, the in-phase/opposite phase discrimination circuit outputs a voltage-down signal to a charge pump. When the phases are opposite to each other, the in-phase/opposite phase discrimination circuit outputs a voltage-up signal to the charge pump. The charge pump generates a voltage signal in response to the voltage-up signal and the voltage-down signal outputted from the in-phase/opposite phase discrimination circuit. The voltage signal is transmitted from the charge pump to a control terminal of the voltage controlled oscillator via a low pass filter. Thus, the frequency of the first local oscillator signal and also the frequency of the second local oscillator signal are controlled in response to the voltage signal generated by the charge pump. This frequency control is designed to provide automatic frequency control (AFC) which enables the frequency of the first and second local oscillator signals to automatically follow the carrier frequency (the center frequency) of the received RF FSK signal.
The selectivity of the direct conversion receiver of Japanese application 6-54005 is determined by the cutoff frequency of the first and second low pass filters which follow the first and second mixers. The cutoff frequency of the first and second low pass filters is fixed. Therefore, in the direct conversion receiver of Japanese application 6-54005, it is difficult to automatically adjust the selectivity in response to signal receiving conditions.
As previously explained, the direct conversion receiver of Japanese application 6-54005 has the AFC function of correcting an error between the frequency of the first and second local oscillator signals and the carrier frequency (the center frequency) of a received RF FSK signal. The AFC can not correct a frequency error having a magnitude corresponding to greater than a passband provided by the receiver selectivity. The passband is fixed since the cutoff frequency of the first and second low pass filters and also the receiver selectively are fixed. Thus, in the direct conversion receiver of Japanese application 6-54005, it is difficult to automatically adjust the greater limit of a frequency error which can be corrected by the AFC.
It is an object of this invention to provide an improved direct conversion receiver.
A first aspect of this invention provides a direct conversion receiver comprising a local oscillator for generating a first local oscillator signal having a variable frequency; a 90xc2x0 phase shifter connected to the first local oscillator for shifting a phase of the first local oscillator signal by 90xc2x0 to convert the first local oscillator signal into a second local oscillator signal; a first mixer connected to the local oscillator for mixing a received RF signal and the first local oscillator signal; a second mixer connected to the 90xc2x0 phase shifter for mixing the received RF signal and the second local oscillator signal; a first low pass filter connected to the first mixer for processing an output signal of the first mixer, the first low pass filter having a controllable cutoff frequency; a second low pass filter connected to the second mixer for processing an output signal of the second mixer, the second low pass filter having a controllable cutoff frequency; a demodulator connected to the first and second low pass filters for recovering an information signal represented by the received RF signal from output signals of the first and second low pass filters; a frequency error detector connected to the demodulator for detecting an error between a frequency of a carrier of the received RF signal and the frequency of the first local oscillator signal in response to the information signal recovered by the demodulator; an oscillator controller connected to the local oscillator and the frequency error detector for controlling the frequency of the first local oscillator signal in response to the error detected by the frequency error detector; and a filter controller connected to the first and second low pass filters and the frequency error detector for controlling the cutoff frequencies of the first and second low pass filters in response to the error detected by the frequency error detector.
A second aspect of this invention is based on the first aspect thereof, and provides a direct conversion receiver wherein the frequency error detector comprises an integrator for integrating a voltage of the information signal over a given time interval longer than a period corresponding to a transmission data rate related to the information signal.
A third aspect of this invention is based on the first aspect thereof, and provides a direct conversion receiver wherein the frequency error detector comprises a peak detector for detecting a peak voltage of the information signal which occurs during a given time interval longer than a period corresponding to a transmission data rate related to the information signal, a valley detector for detecting a valley voltage of the information signal which occurs during the given time interval, and an adder for adding the peak voltage detected by the peak detector and the valley voltage detected by the valley detector.
A fourth aspect of this invention is based on the first aspect thereof, and provides a direct conversion receiver wherein the filter controller comprises means for changing each of the cutoff frequencies of the first and second low pass filters between at least two different values in response to the error detected by the frequency error detector.
A fifth aspect of this invention is based on the first aspect thereof, and provides a direct conversion receiver wherein the filter controller comprises means for continuously varying the cutoff frequencies of the first and second low pass filters in accordance with the error detected by the frequency error detector.
A sixth aspect of this invention provides a direct conversion receiver comprising a local oscillator for generating a first local oscillator signal having a variable frequency; a 90xc2x0 phase shifter connected to the first local oscillator for shifting a phase of the first local oscillator signal by 90xc2x0 to convert the first local oscillator signal into a second local oscillator signal; a first mixer connected to the local oscillator for mixing a received RF signal and the first local oscillator signal; a second mixer connected to the 90xc2x0 phase shifter for mixing the received RF signal and the second local oscillator signal; a first low pass filter connected to the first mixer for processing an output signal of the first mixer, the first low pass filter having a controllable cutoff frequency; a second low pass filter connected to the second mixer for processing an output signal of the second mixer, the second low pass filter having a controllable cutoff frequency; a demodulator connected to the first and second low pass filters for recovering an information signal represented by the received RF signal from output signals of the first and second low pass filters; a frequency error detector connected to the demodulator for detecting an error between a frequency of a carrier of the received RF signal and the frequency of the first local oscillator signal in response to the information signal recovered by the demodulator; an oscillator controller connected to the local oscillator and the frequency error detector for controlling the frequency of the first local oscillator signal in response to the error detected by the frequency error detector; and a signal processor connected to the first and second low pass filters and the demodulator for deciding whether or not a given signal pattern is present in the information signal recovered by the demodulator, and for controlling the cutoff frequencies of the first and second low pass filters in response to a result of the deciding.
A seventh aspect of this invention provides a direct conversion receiver comprising a local oscillator for generating a local oscillator signal; a mixer connected to the local oscillator for mixing a received RF FSK signal and the local oscillator signal; a low pass filter connected to the mixer for processing an output signal of the mixer, the low pass filter having a changeable cutoff frequency; first means for detecting an error between a frequency of a carrier of the received RF FSK signal and a frequency of the local oscillator signal; and second means connected to the low pass filter and the first means for changing the cutoff frequency of the low pass filter in response to the error detected by the first means.
An eighth aspect of this invention is based on the seventh aspect thereof, and provides a direct conversion receiver further comprising third means connected to the local oscillator and the first means for controlling a frequency of the local oscillator signal in response to the error detected by the first means.
A ninth aspect of this invention provides a direct conversion receiver comprising a local oscillator for generating a local oscillator signal; a mixer connected to the local oscillator for mixing a received RF FSK signal and the local oscillator signal; a low pass filter connected to the mixer for processing an output signal of the mixer, the low pass filter having a changeable cutoff frequency; a demodulator connected to the low pass filter for recovering an information signal represented by the received RF FSK signal from an output signal of the low pass filter; and a signal processor connected to the low pass filter and the demodulator for deciding whether or not a given signal pattern is present in the information signal recovered by the demodulator, and for changing the cutoff frequency of the low pass filter in response to a result of the deciding.
A tenth aspect of this invention is based on the ninth aspect thereof, and provides a direct conversion receiver further comprising first means for detecting an error between a frequency of a carrier of the received RF FSK signal and a frequency of the local oscillator signal, and second means connected to the local oscillator and the first means for controlling the frequency of the local oscillator signal in response to the error detected by the first means.